In an ejector cycle shown in FIG. 13, refrigerant flows in this order of a compressor 110→a radiator 120, a nozzle of an ejector 140→a gas-liquid separator 150→the compressor 110. At the same time, refrigerant also flows by an entrainment function of a high-speed drive flow of the refrigerant jetted from the nozzle of the ejector 140, in this order of gas-liquid separator 150→a throttle 161→an evaporator 130→a pressure increasing portion of the ejector 140→the gas-liquid separator 150.
FIG. 14 shows drive flow states in the ejector cycle in FIG. 13 when carbon dioxide is used as the refrigerant. In FIG. 14, P1-P6 indicate refrigerant states at the same positions shown in FIG. 13. Further, the solid line state P1-P6 in FIG. 14 shows a state where a thermal load in the evaporator 130 is larger than the chain-line state P1-P6. Generally, when a pressure loss in a refrigerant passage connecting the evaporator 130 and the ejector 140 is not considered, a pressure in the evaporator 130 is approximately equal to a pressure at the position P3 in the ejector 140.
In a cool-down operation in summer, an outside air temperature is high, and a thermal load (air-conditioning load) of the evaporator 130 is high. In this case, because the outside air temperature for cooling refrigerant in the radiator 120 is high, an enthalpy different in a decompression of the nozzle of the ejector 140 becomes larger, and a pressure increasing amount in a pressure increasing portion of the ejector 140 becomes larger (P4→P5). Thus, the pressure of refrigerant in the gas-liquid separator 150 is increased to a pressure (P6) near the critical pressure of the refrigerant as shown in FIG. 14. Accordingly, as shown in FIG. 14, the pressure of refrigerant to be sucked into the compressor 100 is increased, and the specific enthalpy of the refrigerant flowing into the radiator 120 becomes smaller. Further, because the outside air temperature is high, the refrigerant flowing into the radiator 120 is not sufficiently cooled, and the heat-radiating capacity of the radiator 120 is decreased.
Further, when the pressure of refrigerant flowing into the nozzle of the ejector 140 is higher than the critical pressure as shown in FIG. 14, a pressure increasing amount in the pressure increasing portion of the ejector 140 becomes greatly larger, as compared with a case where the pressure of the refrigerant flowing into the nozzle of the ejector is lower than the critical pressure. Thus, in a super-critical refrigerant cycle where the pressure of refrigerant flowing into the nozzle is higher than the critical pressure, when the outside air temperature is high and the pressure in the evaporator 30 is high in the cool-down operation (rapid cooling operation), the heat radiating capacity of the radiator 120 may be greatly reduced.